Since the nonaqueous electrolyte battery such as lithium ion secondary battery have high energy densities, they are expected to be used for power sources for hybrid vehicles, electric cars, and an uninterruptible power supply for base stations for portable telephone, and the like. Therefore, the nonaqueous electrolyte battery is desired to have other performances such as rapid charge/discharge performances and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge/discharge not only remarkably shortens the charging time but also makes it possible to improve performances of the motive force of a hybrid vehicle and the like and to efficiently recover the regenerative energy of them.
In order to enable rapid charge/discharge, it is necessary that electrons and lithium ions can migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon based material in the negative electrode repeats rapid charge/discharge, this causes dendrite precipitation of metal lithium on the electrode, raising the fear as to heat generation and fires caused by internal short circuits.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in the negative electrode has been developed. Particularly, a battery using a titanium oxide as the negative electrode active material enables stable rapid charge/discharge and also has a longer life than those using a carbonaceous material.
However, titanium oxide has a higher potential than carbonaceous material relative to metal lithium. Further, titanium oxide has a lower capacity per mass. Thus a battery using titanium oxide as the negative electrode active material has a problem that the energy density is lower.
For example, the potential of the electrode using titanium oxide is about 1.5 V based on metal lithium and is nobler than that of the electrode using carbonaceous material. The potential of titanium oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and released and is therefore limited electrochemically. Further, there is the fact that the inserted and released of lithium ions by rapid charge/discharge is possible at an electrode potential as high as about 1.5 V. It is therefore substantially difficult to drop the potential of the electrode to improve energy density.
As to the capacity of the battery per unit mass, the theoretical capacity of titanium dioxide having an anatase structure is about 165 mAh/g and the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is also about 170 mAh/g. On the other hand, the theoretical capacity of a general graphite type electrode material is 385 mAh/g or more. Therefore, the capacity density of titanium oxide is significantly lower than that of the carbon type material. This is due to a reduction in substantial capacity because there are only a small number of equivalent lithium-absorbing sites in the crystal structure and lithium tends to be stabilized in the structure.
In light of this, monoclinic titanium dioxide has recently attracted attention (see R. Marchand, L. Brohan, M. Tournoux, Material Research Bulletin 15, 1129 (1980)). Lithium titanate having a spinel structure such as Li4Ti5O12 can release/insert 3 lithium ions per unit chemical formula. Therefore, the number of lithium ions per titanium ion is ⅗ and a theoretical maximum of 0.6. On the other hand, in monoclinic titanium dioxide, the number of lithium ions per titanium ion which can be released/inserted is a maximum of 1.0. Accordingly, the theoretical capacity of monoclinic titanium dioxide is about 330 mAh/g. Therefore, it is expected that monoclinic titanium dioxide may be used as a high-capacity negative electrode active material.
For example, JP-A 2008-34368 (KOKAI) discloses a lithium ion storage battery using titanium oxide TiO2 having a bronze structure as the negative electrode active material. JP-A 2008-34368 (KOKAI) discloses that the substantial capacity of a lithium ion storage battery using the titanium oxide TiO2 as the electrode active material and a lithium metal as the counter electrode is about 200 mAh/g (for example, Paragraph 0029 and FIG. 4).
JP-A 2008-117625 (KOKAI) discloses a lithium secondary battery using, as the active material, titanium dioxide having crystal structure of a bronze type titanic acid. JP-A 2008-117625 (KOKAI) discloses that a lithium secondary battery (coin type cell) using the titanium dioxide as the active material and a lithium metal as the counter electrode has an initial insertion and release capacity of 160 to 170 mAh/g based on the mass of the active material (for example, Paragraphs 0053 and 0057).
Although, the theoretical capacity in the case of using monoclinic titanium dioxide as the active material is about 330 mAh/g, the substantial capacities disclosed in JP-A 2008-34368 (KOKAI) and JP-A 2008-117625 (KOKAI) are significantly lower than the theoretical capacity. Therefore, if the titanium dioxide described in JP-A 2008-34368 (KOKAI) or JP-A 2008-117625 (KOKAI) are used as the active material, it is difficult to raise the capacity further as compared, for example, with the case of using lithium titanate having a spinel structure which has the theoretical capacity of 170 mAh/g.